Semiconductor device with cell trench structures and a contact structure

ABSTRACT

A semiconductor device includes first and second cell trench structures extending from a first surface into a semiconductor body, a first semiconductor mesa separating the cell trench structures. The first cell trench structure includes a first buried electrode and a first insulator layer. A first vertical section of the first insulator layer separates the first buried electrode from the first semiconductor mesa. The first semiconductor mesa includes a source zone of a first conductivity type directly adjoining the first surface. The semiconductor device further includes a capping layer on the first surface and a contact structure having a first section in an opening of the capping layer and a second section in the first semiconductor mesa or between the first semiconductor mesa and the first buried electrode. A lateral net impurity concentration of the source zone parallel to the first surface increases in the direction of the contact structure.

BACKGROUND

Semiconductor devices based on vertical IGFET (insulated gate fieldeffect transistor) cells include cell trench structures with buriedelectrodes and semiconductor mesas between the cell trench structures.Typically, a photolithographic mask defines placement and size of thecell trench structures, another photolithographic mask defines placementand size of impurity zones in the semiconductor mesas and a furtherphotolithographic mask defines contact structures providing electriccontacts to the impurity zones. Other approaches rely on forming thecontact structures self-aligned to the cell trench structures. It isdesirable to provide semiconductor devices with narrow semiconductormesas and small distances between neighboring cell trench structures ina reliable way and at low costs.

SUMMARY

According to an embodiment, a method of manufacturing a semiconductordevice includes forming a semiconductor mesa in a semiconductor layerbetween a first cell trench structure and a second cell trench structureextending from a first surface into the semiconductor layer. An openingis formed in a capping layer formed on the first surface, wherein theopening exposes at least a portion of the semiconductor mesa. Throughthe opening impurities of a first conductivity type are introduced intothe exposed portion of the semiconductor mesa. A recess defined by theopening is formed.

According to another embodiment a semiconductor device includes firstand second cell trench structures extending from a first surface into asemiconductor body. A first semiconductor mesa separates the first andsecond cell trench structures. The first cell trench structure includesa first buried electrode and a first insulator layer. A first verticalsection of the first insulator layer separates the first buriedelectrode from the first semiconductor mesa. The first semiconductormesa includes a source zone of a first conductivity type directlyadjoining the first surface. The semiconductor device further includes acapping layer on the first surface. A contact structure includes a firstsection in an opening of the capping layer and a second section in thefirst semiconductor mesa or between the first semiconductor mesa and thefirst buried electrode. A lateral net impurity concentration of thesource zone parallel to the first surface increases in the direction ofthe contact structure.

According to another embodiment, a method of manufacturing asemiconductor device includes forming first and second cell trenchstructures that extend from a first surface into a semiconductorsubstrate. The first cell trench structure includes a first buriedelectrode and a first insulator layer between the first buried electrodeand a semiconductor mesa separating the first and second cell trenchstructures. A capping layer is formed that covers the first surface. Thecapping layer is patterned to form an opening that exposes a firstvertical section of the first insulator layer at the first surface.Impurities for forming a source zone of a first conductivity type areintroduced into an exposed portion of the semiconductor mesa through theopening. An exposed portion of the first insulator layer is removed toform a recess between the semiconductor mesa and the first buriedelectrode using the patterned capping layer as an etch mask.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description and onviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification. The drawings illustrate the embodiments ofthe present invention and together with the description serve to explainprinciples of the embodiment. Other embodiments and intended advantageswill be readily appreciated as they become better understood byreference to the following detailed description.

FIG. 1A is a schematic cross-sectional view of a portion of asemiconductor substrate for illustrating a method of manufacturing asemiconductor device according to an embodiment, after forming anopening in a capping layer.

FIG. 1B is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 1A after narrowing the opening by a reflow ofthe capping layer.

FIG. 1C is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 1B after forming a recess for a contactstructure.

FIG. 1D is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 1C after providing contact structures fillingthe opening and the recess.

FIG. 2A is a schematic cross-sectional view of a portion of asemiconductor substrate after providing a capping layer for illustratinga method of manufacturing a semiconductor device according to anembodiment providing a single etch mask for defining source zones andcontacts.

FIG. 2B is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 2A after forming openings in the capping layerby using the etch mask and introducing impurities for forming sourcezones by using the etch mask.

FIG. 2C is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 2B after providing a stray oxide.

FIG. 2D is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 2C after forming recesses between first celltrench structures and first semiconductor mesas and introducingimpurities for contact zones through the recesses.

FIG. 2E is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 2D after providing contact structures in theopenings and recesses.

FIG. 3A shows a portion of a semiconductor substrate after providingrecesses between a first buried electrode and first semiconductor mesas.

FIG. 3B shows the semiconductor substrate portion of FIG. 3A afterwidening the recesses.

FIG. 3C illustrates the semiconductor substrate portion of FIG. 3B afterproviding contact structures in the openings and widened recesses.

FIG. 4A is a schematic perspective view of a portion of semiconductordevice in accordance with an embodiment related to an IGBT.

FIG. 4B illustrates a cross section of the semiconductor devices of FIG.4A along section line B.

FIG. 4C illustrates a cross section of the semiconductor devices of FIG.4A along section line C.

FIG. 4D illustrates a cross section of the semiconductor devices of FIG.4A along section line D.

FIG. 5 is a schematic plan view of a portion of a semiconductor devicein accordance with an embodiment providing laterally patterned sourcezones.

FIG. 6A is a schematic plan view of a portion of a semiconductor devicein accordance with an embodiment providing a reinforcement implant in anedge area.

FIG. 6B is a schematic plan view of a portion of a semiconductor deviceaccording to an embodiment providing auxiliary contacts in an edge area.

FIG. 6C is a schematic plan view of a portion of a semiconductor deviceaccording to another embodiment providing an auxiliary contact in anedge area.

FIG. 7A is a schematic plan view of a portion of a semiconductor deviceaccording to an embodiment providing a partially buried electrodestructure.

FIG. 7B is a schematic cross-sectional view of the semiconductor deviceportion of FIG. 7A along line B-B.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which are shownby way of illustrations specific embodiments in which the invention maybe practiced. It is to be understood that other embodiments may beutilized and structural or logical changes may be made without departingfrom the scope of the present invention. For example, featuresillustrated or described for one embodiment can be used on or inconjunction with other embodiments to yield yet a further embodiment. Itis intended that the present invention includes such modifications andvariations. The examples are described using specific language thatshould not be construed as limiting the scope of the appending claims.The drawings are not scaled and are for illustrative purposes only. Forclarity, the same elements have been designated by correspondingreferences in the different drawings if not stated otherwise.

The terms “having”, “containing”, “including”, “comprising” and the likeare open and the terms indicate the presence of stated structures,elements or features but not preclude additional elements or features.The articles “a”, “an” and “the” are intended to include the plural aswell as the singular, unless the context clearly indicates otherwise.The term “electrically connected” describes a permanent low-ohmicconnection between electrically connected elements, for example a directcontact between the concerned elements or a low-ohmic connection via ametal and/or highly doped semiconductor. The term “electrically coupled”includes that one or more intervening element(s) adapted for signaltransmission may be provided between the electrically coupled elements,for example elements that are controllable to temporarily provide alow-ohmic connection in a first state and a high-ohmic electricdecoupling in a second state.

The Figures illustrate relative doping concentrations by indicating “−”or “+” next to the doping type “n” or “p”. For example, “n⁻” means adoping concentration that is lower than the doping concentration of an“n”-doping region while an “n⁺”-doping region has a higher dopingconcentration than an “n”-doping region. Doping regions of the samerelative doping concentration do not necessarily have the same absolutedoping concentration. For example, two different “n”-doping regions mayhave the same or different absolute doping concentrations.

FIGS. 1A to 1D refer to a semiconductor substrate 500 a consisting of orcontaining a semiconductor layer 100 a of a single-crystallinesemiconductor material. The single-crystalline semiconductor materialmay be silicon Si, silicon carbide SiC, germanium Ge, a silicongermanium crystal SiGe, gallium nitride GaN or gallium arsenide GaAs.The semiconductor substrate 500 a may be a silicon wafer from which aplurality of identical semiconductor dies is obtained. The semiconductorlayer 100 a has a planar first surface 101 and a second surface 102parallel to the first surface 101. The normal to the first and secondsurfaces 101, 102 defines a vertical direction and directions orthogonalto the vertical direction are lateral directions.

A drift layer 120 of a first conductivity type may be formed between thefirst and second surfaces 101, 102. A heavily doped pedestal layer 130,which may have the first or second conductivity type or which mayinclude zones of both impurity types, may separate the drift layer 120from the second surface 102. A body layer of a second conductivity type,which is the opposite of the first conductivity type, separates thedrift layer 120 from the first surface 101. Pn junctions between thebody layer and the drift layer 120 may be parallel to the first surface101

The first conductivity type may be the n type and the secondconductivity type may be the p type as illustrated in the Figures.According to other embodiments the first conductivity type may be the ptype and the second conductivity type may be the n type. Outside theillustrated portion, the semiconductor layer 100 a may include furtherimpurity zones, intrinsic zones, as well as dielectric and conductivestructures that may be configured to form electronic components orcircuits.

A first cell trench structure 510 and a second cell trench structure 520extend from the first surface 101 into the semiconductor layer 100 a,wherein buried edges of the first and second cell trench structures 510,520 have a greater distance to the first surface 101 than a pn junctionbetween the body layer and the drift layer 120. The cell trenchstructures 510, 520 partition the body layer into segments such that asemiconductor mesa 150 between the first and second cell trenchstructures 510, 520 has a layered structure with a body zone 115 betweenthe first surface 101 and a portion of the drift layer 120.

The first cell trench structure 510 includes at least a first buriedelectrode 515 and a first insulator layer 516 separating the firstburied electrode 515 from the semiconductor layer 100 a. The second celltrench structure 520 includes a second buried electrode 525 and a secondinsulator layer 526 separating the second buried electrode 525 from thesemiconductor layer 100 a. At least one of the first and second celltrench structures 510, 520 may include a further buried electrodedielectrically insulated from the respective first or second buriedelectrode 515, 525. The second buried electrode 525 may be connected toa gate terminal of a semiconductor switching device whose semiconductordie is obtained from the finalized semiconductor substrate 500 a.

The first and second cell trench structures 510, 520 may have the samevertical and lateral dimensions. According to other embodiments thefirst cell trench structure 510 may be wider or narrower than the secondcell trench structure 520. Alternatively or in addition, the verticalextension of the first cell trench structure 510 exceeds or falls belowthe vertical extension of the second cell trench structure 520.According to an embodiment, the vertical extension of both the first andthe second cell trench structures 510, 520 may be in a range from 500 nmto 20 μm, e.g. in a range from 2 μm to 7 μm.

The first and second buried electrodes 515, 525 and, if applicable, thefurther buried electrode(s) may be provided from one or more conductivematerials including polycrystalline silicon (polysilicon), which may beheavily doped, metal silicides, carbon C, metals, e.g. copper ortungsten, metal alloys, metal nitrides, metal silicides or other metalcompounds, e.g. titanium nitride TiN, titanium tungstenide TiW, tantalumnitride TaN and others. For example, the first, the second, or bothburied electrodes 515, 525 have a layered structure including two ormore layers of the above-mentioned materials. The first and secondburied electrodes 515, 525 may have the same structure and may containthe same materials or may have different structures and/or containdifferent materials.

The first and second insulator layers 516, 526 may have the samethickness or may have different thicknesses. For example, the firstinsulator layer 516 may be thicker than the second insulator layer 526.The first and second insulator layers 516, 526 may be based on the samematerials or may consist of or may include different materials such assemiconductor oxides, e.g. silicon oxide, silicon nitride, alumina, andhafnium oxide, by way of example. According to an embodiment, at leastone of the first and second insulator layers 516, 526 has a layeredstructure including one or more different dielectric materials. Athickness of the first and second insulator layers may be between 30 nmand 200 nm, e.g. in the range between 80 nm and 120 nm.

The first and second buried electrodes 515, 525 may be electricallyconnected to each other. According to the illustrated embodiment thefirst and second buried electrodes 515, 525 are electrically separatedfrom each other and can be connected to different signals or potentials.A potential applied to the second buried electrode 525 may control thecharge carrier distribution in the adjoining body zone 115 such thatalong the second insulator layer 526 a conductive inversion channel ofminority charge carriers may be formed when the potential applied to thesecond buried electrode 525 exceeds or falls below a predefinedthreshold voltage. A section of the second insulator layer 526 adjoiningthe body zone 115 is effective as a gate dielectric.

A capping layer 220 is provided on the first surface 101 and covers thefirst and second cell trench structures 510, 520 as well as thesemiconductor mesa 150. The capping layer 220 may include one or moredielectric layers, each layer provided, for example, from depositedsemiconductor oxide, for example a silicon oxide generated by using TEOS(tetraethyl orthosilicate) as precursor material, other silicon oxides,silicon nitride, or silicon oxynitride. According to an embodiment thecapping layer 220 includes or consists of a layer of a silicate glass,e.g., PSG (phosphorus silicate glass), BSG (boron silicate glass), orBPSG (boron phosphorus silicate glass). The thickness of the cappinglayer 220 may be approximately uniform and may range from about 100 nmto 1 μm, by way of example.

A mask layer may be patterned by photolithography to obtain an etch maskwith a mask opening. For example, a photo resist layer may be depositedand patterned by photolithography to obtain the etch mask. According toanother embodiment, a patterned photo resist layer may be used topattern a hard mask layer provided above the capping layer 220 and thepatterned hard mask layer may form the etch mask. The mask opening inthe etch mask selectively exposes portions of the capping layer 220,e.g., in the vertical projection of a mesa portion spaced from bothadjoining cell trench structures 510, 520 or in the vertical projectionof first vertical sections of the first insulator layers 516, whereinthe first vertical sections adjoins the semiconductor mesa 150 thatseparates the first and second cell trench structures 510, 520.

An alignment of the mask opening with respect to the second cell trenchstructure 520 is chosen to ensure a sufficient diffusion of impurities,which are later introduced into the exposed portion of the semiconductormesa 150, up to the second cell trench structure 520. The alignmentposition is subject to the mesa width to ensure a correct positioning ofthe mask opening 405 and to prevent an etching of the second insulatorlayer 526 of the second cell trench structure 520. The mask opening istransferred into the capping layer 220 in a predominantly anisotropicetch process using the etch mask, wherein an opening 305 x is formed inthe capping layer 220. The etch process may include an endpointdetection sensitive to reaching the semiconductor mesas 150. The opening305 x exposes a portion of the semiconductor mesa spaced from both celltrench structures 510, 520.

Impurities 411 of the first conductivity type are introduced into theexposed portion of the semiconductor mesa 150 between the first andsecond cell trench structures 510, 520 through the first surface 101,e.g., by outdiffusion from the solid or gaseous phase or by way of animplant. An implant angle between an implant beam and the normal to thefirst surface 101 may be greater than 7 degrees, or, at least 30 degreesand at most 60 degrees, wherein the implant beam is directed to theadjoining second cell trench structure 520.

The impurities may be introduced with no or only a low thermal budgetapplied to the semiconductor substrate 500 a after formation of theopening 305 x in the capping layer 220 such that the opening 305 x mayhave approximately straight, e.g., perpendicular side walls.

FIG. 1A shows that remnant portions of the capping layer 220 cover thefirst and second cell trench structures 510, 520 as well as portions ofthe semiconductor mesa 150 directly adjoining the first and second celltrench structures 510, 520. The opening 305 x in the capping layer 220exposes a central portion of the semiconductor mesa 150 spaced from theadjoining cell trench structures 510, 520. A first offset x1 between theopening 305 x and the second cell trench structure 520 may be in a rangefrom 0 to 150 nm, by way of example.

An implant zone 110 a formed by the introduced impurities directlyadjoins the first surface 101 in the exposed portion of thesemiconductor mesa 150. The implant zone 110 a may or may not undercut asection of the remnant portion of the capping layer 220 that directlyadjoins the opening 305 x at the side of the second cell trenchstructure 520.

The etch mask may be removed or consumed and implant damages in thesemiconductor substrate 500 a may be annealed. A tempering process at atemperature above the reflow temperature of at least one of thematerials of the capping layer may combine the outdiffusion of implantedimpurities from the implant zone 110 a with a reflow of the cappinglayer 220. In addition, the cross-sectional area of the opening 305 x isnarrowed.

For example, a thin auxiliary layer of the material of the capping layer200 or a similar material may be deposited before the tempering process,wherein the thin auxiliary layer covers the portion of the semiconductormesa 150 exposed by the opening 305 x. During the tempering processmaterial from the capping layer 220 of FIG. 1A flows into the area ofthe opening 305 x. After the tempering process an isotropic etch mayremove the thin auxiliary layer and may uniformly thin the capping layer220 after the reflow to expose an area of the semiconductor mesa 150which is smaller than the lateral cross-sectional area of the opening305 x in FIG. 1A and which is formed self-aligned to the opening 305 x.

According to the illustrated embodiment, a spacer layer 221 is depositedbefore or after the tempering process. The spacer layer 221 may beformed from a dielectric material, e.g., a silicon oxide based on TEOS(tetraethyl orthosilicate) as precursor.

As shown in FIG. 1B the spacer layer 221 narrows the opening 305 x,wherein the narrowed opening 305 x may have a second offset x2 to thesecond cell trench structure 520 that is greater than the first offsetx1 illustrated in FIG. 1A by the thickness of the spacer layer 221,e.g., some ten nanometers. A source zone 110 obtained by diffusion fromthe implanted zone 110 a of FIG. 1A directly adjoins the second celltrench structure 520. A plurality of spatially separated source zonesmay be formed along a lateral direction perpendicular to thecross-sectional plane. Due to the lateral diffusion, a lateral impurityconcentration profile in the source zone 110 decreases into thedirection of the second cell trench structure 520. In the body zone 115a maximum impurity concentration of impurities of the secondconductivity type may have a distance to the first surface 101 that isgreater than the distance between the first surface 101 and the pnjunction formed between the source zone 110 and the body zone 115.

An anisotropic spacer etch may remove horizontal portions of the spacerlayer 221 on the capping layer 220 and on the first surface 101 in theopening 305 x. The spacer etch exposes an area of the semiconductor mesa150 which is smaller than the lateral cross-sectional area of theopening 305 x in FIG. 1A and which is formed self-aligned to the opening305 x. A recess 305 y is etched into a portion of the semiconductor mesa150 exposed by the narrowed opening 305 x down to a second distance tothe first surface 101, which is greater than a first distance betweenthe first surface 101 and the pn junction between the source and bodyzones 110, 115 and which is smaller than a third distance between thefirst surface 101 and the pn junction between the body zones 115 and thedrift layer 120. The second distance may be at least 200 nm and at most1 μm, e.g. between 400 μm and 600 μm.

FIG. 1C shows a spacer 221 a obtained by the spacer etch from the spacerlayer 221 of FIG. 1B as well as the resulting recess 305 y in thesemiconductor mesa 150. The spacer 221 a extends along the sidewall ofthe narrowed opening 305 x. The recess 305 y is self-aligned withrespect to the source implant. The spacer 221 a facilitates the use ofone single photolithographic mask for the definition of both sourcezones 110 and source/body contacts without raising the alignmentrequirements. According to an embodiment, the width of the semiconductormesa 150 may be in a range from 400 nm to 800 nm at recess widths from100 nm to 300 nm

Impurities of the second conductivity type may be introduced into thesemiconductor mesa 150 through the recess 305 y to form heavily dopedcontact zones 117. One or more conductive materials may be deposited toform a first electrode structure 310 on the side of the semiconductorsubstrate 500 a defined by the first surface 101 as well as a contactstructure 315 providing a source/body contact electrically connectingthe first electrode structure 310 with the body zone 115 and the sourcezone 110 in the semiconductor mesa 150. Providing the first electrodestructure 310 may include successive deposition of one or moreconductive materials.

According to an embodiment, a barrier layer 311 having a uniformthickness in the range of 5 nm to 100 nm may be deposited. The barrierlayer 311 may prevent metal atoms from diffusing into the semiconductorsubstrate 500 a and may be a layer of titanium nitride TiN, tantalumnitride TaN, titanium tungstenide TiW, titanium Ti or tantalum Ta, ormay include more than one of these materials.

A main layer 312 may be deposited on the barrier layer 311. The mainlayer 312 may consist of or contain tungsten or tungsten based metalssuch as titanium tungstenide TiW, heavily doped polysilicon, carbon C,aluminum Al, copper Cu or alloys of aluminum and copper, such as AlCu orAlSiCu. At least one of the layers may be provided with a porousstructure or may be deposited in a way to form voids or small cavitieswithin the recess 305 y and/or the opening 305 x. Voids and cavities inthe recess 305 y and the opening 305 x may reduce mechanical stress inthe semiconductor substrate 500 a.

FIG. 1D shows the first electrode structure 310 including the barrierlayer 311 and the main layer 312. The thickness of the barrier layer 311may be less than a half of the width of the recess 305 y in FIG. 1C.According to another embodiment, the barrier layer 311 fills the recess305 y completely. The materials of the main layer 312 and the barrierlayer 311 may fill the openings 305 x in the capping layer 220 and therecesses 305 y in the semiconductor portion 100 completely to form solidcontact structures 315. A contact zone 117 is formed in thesemiconductor mesa 150 between the contact structure 315 and the bodyzone 115

FIGS. 2A to 2E refer to a semiconductor substrate 500 a consisting of orcontaining a semiconductor layer 100 a of a single-crystallinesemiconductor material. The single-crystalline semiconductor materialmay be silicon Si, silicon carbide SiC, germanium Ge, a silicongermanium crystal SiGe, gallium nitride GaN or gallium arsenide GaAs.The semiconductor substrate 500 a may be a silicon wafer from which aplurality of identical semiconductor dies is obtained. The semiconductorlayer 100 a has a planar first surface 101 and a second surface 102parallel to the first surface 101. The normal to the first and secondsurfaces 101, 102 defines a vertical direction and directions orthogonalto the vertical direction are lateral directions.

A drift layer 120 of a first conductivity type may be formed between thefirst and second surfaces 101, 102. A heavily doped pedestal layer 130,which may have the first or second conductivity type or which mayinclude zones of both impurity types, may separate the drift layer 120from the second surface 102. A body layer 115 x of a second conductivitytype, which is the opposite of the first conductivity type, separatesthe drift layer 120 from the first surface 101. Pn junctions between thebody and drift layers 115 x, 120 may be parallel to the first surface101.

The first conductivity type may be the n type and the secondconductivity type may be the p type as illustrated in the Figures.According to other embodiments the first conductivity type may be the ptype and the second conductivity type may be the n type. Outside theillustrated portion, the semiconductor layer 100 a may include furtherimpurity zones, intrinsic zones, as well as dielectric and conductivestructures that may be configured to form electronic components orcircuits.

First and second cell trench structures 510, 520 extend from the firstsurface 101 into the semiconductor layer 100 a wherein buried edges ofthe first and second cell trench structures 510, 520 have a greaterdistance to the first surface 101 than the pn junctions between the bodylayer 115 x drift layer 120. The cell trench structures 510, 520partition the body layer 115 x into body zones 115 such thatsemiconductor mesas 150 between the cell trench structures 510, 520 havea layered structure with body zones 115 directly adjoining the firstsurface 101 in first portions of the semiconductor mesas 150 oriented tothe first surface 101 and sections of the drift layer 120 in secondportions oriented to the second surface 102.

The first cell trench structures 510 include at least a first buriedelectrode 515 and a first insulator layer 516 separating the firstburied electrode 515 from the semiconductor material of thesemiconductor substrate 500 a outside the first and second cell trenchstructures 510, 520.

Each second cell trench structure 520 includes a second buried electrode525 and a second insulator layer 526 separating the second buriedelectrode 525 from the semiconductor material of the semiconductorsubstrate 500 a outside the cell trench structures 510, 520. At leastone of the first and second cell trench structures 510, 520 may includea further buried electrode dielectrically insulated from the respectivefirst or second buried electrode 515, 525.

The first and second cell trench structures 510, 520 may have the samevertical and lateral dimensions. According to other embodiments thefirst cell trench structures 510 are wider or narrower than the secondcell trench structures 520. Alternatively or in addition, the verticalextension of the first cell trench structures 510 exceeds or falls belowthe vertical extension of the second cell trench structures 520.According to an embodiment, the vertical extension of both the first andthe second cell trench structures 510, 520 may be in a range from 500 nmto 20 μm, e.g. in a range from 2 μm to 7 μm.

The first and second buried electrodes 515, 525 and, if applicable, thefurther buried electrode(s) may be provided from one or more conductivematerials including polycrystalline silicon (polysilicon), which may beheavily doped, metal silicides, carbon C, metals, e.g. copper ortungsten, metal alloys, metal nitrides, metal silicides or other metalcompounds, e.g. titanium nitride TiN, titanium tungstenide TiW, tantalumnitride TaN and others. For example, the first, the second, or bothburied electrodes 515, 516 have a layered structure including two ormore layers of the above-mentioned materials. The first and secondburied electrodes 515, 516 may have the same structure and may containthe same materials or may have different structures and/or containdifferent materials.

The first and second insulator layers 516, 526 may have the samethickness or may have different thicknesses. For example, the firstinsulator layer 516 may be thicker than the second insulator layer 526.The first and second insulator layers 516, 526 may be based on the samematerials or may consist of or may include different materials such assemiconductor oxides, e.g. silicon oxide, silicon nitride, alumina, andhafnium oxide, by way of example. According to an embodiment, at leastone of the first and second insulator layers 516, 526 has a layeredstructure including one or more different dielectric materials. Athickness of the first and second insulator layers may be between 30 nmand 200 nm, e.g. in the range between 80 nm and 120 nm.

The first and second buried electrodes 515, 525 may be electricallyconnected to each other. According to the illustrated embodiment thefirst and second buried electrodes 515, 525 are electrically separatedfrom each other and can be connected to different signals or potentials.A potential applied to the second buried electrodes 525 may accumulateminority charge carriers in the adjoining body zones 115 such that alongthe second insulator layers 526 conductive channels for the minoritycharge carriers may be formed when the potential applied to the secondburied electrodes 525 exceeds or falls below a predefined thresholdvoltage. Thereby sections of the second insulator layers 526 adjoiningthe body zones 115 are effective as gate dielectrics. A capping layer220 is provided on the first surface 101.

FIG. 2A shows the capping layer 220 covering the first and second celltrench structures 510, 520 and the semiconductor mesas 150 between thefirst and second cell trench structures 510, 520. The capping layer 220includes one or more dielectric layers, each layer provided, forexample, from deposited semiconductor oxide, for example a silicon oxidegenerated by using TEOS as precursor material, other silicon oxides,silicon nitride, or silicon oxynitride. The thickness of the cappinglayer 220 may be approximately uniform and may range from about 100 nmto 1 μm, by way of example.

A mask layer may be patterned by photolithography to obtain an etch mask410 with mask openings 405. For example, a photo resist layer may bedeposited and patterned by photolithography to obtain the etch mask 410.According to another embodiment, a patterned photo resist layer may beused to pattern a hard mask layer provided above the capping layer 220and the patterned hard mask layer may form the etch mask 410. The maskopenings 405 in the etch mask 410 selectively expose portions of thecapping layer 220 in the vertical projection of first vertical sectionsof the first insulator layers 516, wherein the first vertical sectionsmay adjoin such semiconductor mesas 150 that separate first and secondcell trench structures 510, 520. An alignment of the mask openings 405with respect to the second cell trench structures 520 is chosen toensure a sufficient diffusion of impurities, which are later introducedinto the exposed semiconductor mesas 150, up to the second cell trenchstructures 520. The alignment position is subject to the mesa width toensure a correct positioning of the mask openings 405 and to prevent anetching of the second insulator layer 526 of the second cell trenchstructure 520.

Using the etch mask 410 a predominantly anisotropic etch recessesexposed portions of the capping layer 220. The etch process may includean endpoint detection sensitive to reaching at least one of thesemiconductor mesas 150, the first vertical sections of the firstinsulator layer 516, and the first buried electrode 515. The endpointdetection may evaluate an optical signal.

Impurities 411 of the first conductivity type are introduced intoexposed semiconductor mesas 150 between the first and second cell trenchstructures 510, 520 through exposed sections of the first surface 101,e.g., by outdiffusion from the solid or gaseous phase or by way of animplant. The impurities may be introduced with no or only a low thermalbudget applied after formation of openings 305 x in the capping layer220 such that the openings 305 x may have straight, e.g., perpendicularside walls. An implant angle α between the normal and an implant beammay be greater than 7 degrees, e.g., at least 30 degrees and at most 60degrees, wherein within the mask openings 405 the implant beam isdirected to the adjoining second cell trench structure 520.

FIG. 2B shows the mask openings 405 in the etch mask 410 as well as theopenings 305 x in the capping layer 220 exposing the first verticalsections of the first insulator layers 516, wherein the first verticalsections adjoin such semiconductor mesas 150 that separate first andsecond cell trench structures 510, 520. The mask openings 405 and theopenings 305 x also expose portions of the semiconductor mesas 150directly adjoining the concerned sections of the first insulator layers516 as well as portions of the first buried electrodes 515 directlyadjoining the concerned sections of the first insulator layers 516. Theetch mask 410 covers portions of the capping layer 220 in the verticalprojection of the second cell trench structures 520 as well as portionsof the capping layer 220 in the vertical projection of second verticalsections of the first insulator layers 516 adjoining semiconductor mesas150 between first cell trench structures 510.

An implant zone 110 a directly adjoins the first surface 101 in theexposed semiconductor mesa 150. The implant zone 110 a may undercut aportion of the patterned capping layer 220 that covers the secondinsulator layer 526 of the adjoining second cell trench structure 520.

The etch mask 410 may be removed or consumed. Before the semiconductorsubstrate 500 a may be annealed to cure implant damages and to diffusethe implanted impurities a stray oxide 222, e.g., a thermal oxide, maybe formed on the exposed sections of the first surface 101.

FIG. 2C shows the stray oxide 222 covering the exposed surface sectionsof the semiconductor mesas 150 as well as a source zone 110 formed byannealing implant damages and by diffusing the implanted impurities ofthe implant zone 110 a of FIG. 5B. The material of the capping layer 220may flow to some degree such that the openings 305 x may be slightlynarrowed. The flow of the material of the capping layer 220 may be usedto decrease a distance between the implant zones 110 a to the secondcell trench structures 520 at a given alignment target. The source zones110 form pn junctions with the body zones at a first distance to thefirst surface 101. The mask openings 405 of FIG. 2B define both thesource zones 110 and the openings 305 x for source/body contacts. As aresult, one photolithography is saved.

Within the body zones 115 a maximum concentration of impurities of thesecond conductivity type is at a fourth distance to the first surface101 which is greater than the first distance between the first surface101 and the finalized source zones 110 such that a slight de-adjustmentof the openings 305 x does not significantly affect the threshold deviceof the concerned IGFET cell. For example, an implant energy for formingthe body zones 115 by an implant through the first surface 101 may beabout 150 keV resulting in a distance of the maximum concentration ofimpurities of the second conductivity type to the first surface 101 ofabout 200 nm to 800 nm, e.g., 300 nm to 600 nm. According to anembodiment a distance between the first surface 101 and the maximumconcentration of impurities of the second conductivity type is in arange from 400 nm to 600 nm.

After the anneal, the exposed first insulator layer 516 may beover-etched for a predefined time to recess the exposed first verticalsections of the first insulator layers 516. The recess etch removesexposed portions of the first insulator layers 516 up to a seconddistance to the first surface 101, which is greater than the firstdistance between the first surface 101 and the pn junction between thesource and body zones 110, 115 and which is smaller than a thirddistance between the first surface 101 and the pn junction between thebody zones 115 and the drift layer 120. The second distance may be atleast 200 nm and at most 1 μm, e.g. between 400 μm and 600 μm. Thematerial of the first insulator layer 516 is recessed at a removal ratethat may be at least five times the removal rate for the semiconductormaterial and/or the material of the first buried electrode 515.

Impurities of the second conductivity type may be introduced into theexposed semiconductor mesa 150 through the recess 305 y, e.g., by anangled implant. According to an embodiment, BF₂ is implanted to formheavily doped contact zones 117 providing reliable ohmicmetal-to-semiconductor contacts for the body zones 115. The BF₂ implantmay be activated by an RTA (rapid thermal anneal), which yields adiffusion in the range of 100 nm allowing to reduce the mesa width downto about 200 nm.

FIG. 2D shows the resulting recesses 305 y between the concerned firstburied electrodes 515 and the concerned semiconductor mesas 150. Due tothe selectivity of the etching the recesses are self-aligned to theadjoining first cell trench structures 510 and the adjoiningsemiconductor mesas 150. In the layout, the width of the semiconductormesas 150 may be reduced, e.g. to below 400 nm. The contact zones 117extend along the recesses 305 y.

One or more conductive materials are deposited to form a first electrodestructure 310 on the side of the semiconductor layer 100 a defined bythe first surface 101 as well as contact structures 315 electricallyconnecting the first electrode structure 310 with the first buriedelectrodes 515, the body zones 115 and the source zones 110 of thesemiconductor mesas 150 that separate first and second cell trenchstructures 510, 520. Providing the first electrode structure 310 mayinclude successive deposition of one or more conductive materials.

According to an embodiment, a barrier layer 311 having a uniformthickness in the range of 5 nm to 100 nm may be deposited. The barrierlayer 311 may prevent metal atoms from diffusing into the semiconductorsubstrate 500 a and may be a layer of titanium nitride TiN, tantalumnitride TaN, titanium tungstenide TiW, titanium Ti or tantalum Ta, ormay include these materials.

A main layer 312 may be deposited on the barrier layer 311. The mainlayer 312 may consist of or contain tungsten or tungsten based metalssuch as titanium tungstenide TiW, heavily doped polysilicon, carbon C,aluminum Al, copper Cu or alloys of aluminum and copper, such as AlCu orAlSiCu. At least one of the layers may be provided with a porousstructure or may be deposited in a way to form voids or small cavitieswithin the recesses 305 y and/or the openings 305 x. Voids and cavitiesin the recesses 305 y and openings 305 x reduce mechanical stress.

FIG. 2E shows the first electrode structure 310 including the barrierlayer 311 and the main layer 312. The thickness of the barrier layer 311may be less than a half of the width of the recess 305 y in FIG. 1C.According to another embodiment, the barrier layer 311 fills the recess305 y completely. The materials of the main layer 312 and the barrierlayer 311 may fill the openings 305 x in the capping layer 220 and therecesses 305 y in the semiconductor portion 100 completely to form solidcontact structures 315 as shown in FIG. 2E. According to otherembodiments, the main and/or barrier layers 312, 311 may be realized asporous layers or may deposited to form cavities, wherein the porousstructure and/or the cavities may reduce thermo-mechanical stress. Themethod may be used for all of the above discussed semiconductor devices.

FIGS. 3A to 3C refer to an embodiment that includes a widening of therecesses 305 y. In first sections of first insulator layers 516 betweenfirst buried electrodes 515 and such semiconductor mesas 150 that areformed between first and second cell trench structures 510, 520 and thatare exposed by openings 305 x in a capping layer 220, recesses 305 y maybe formed between the first buried electrodes 515 and the concernedsemiconductor mesas 150. An etch selectivity at which the material ofthe buried first electrode 515 is removed with respect to the materialof the semiconductor mesa 150 may be at least 5:1. The etch mask may beremoved.

FIG. 3A shows the openings 305 x in the capping layer 220 and therecesses 305 y between the first buried electrodes 515 and the concernedsemiconductor mesas 150. Contact openings 305 include an opening 305 xand a recess 305 y, respectively. A first etch step forms the opening305 x in the capping layer 220 and may stop at the first surface 101. Asecond etch step that may use the same etch process over-etches theexposed first insulator layers 516 for a predetermined time. Using adifferent etch process, a third etch step widens at least the openingsof the recesses 305 y at the expense of either the adjoiningsemiconductor mesas 150 or the adjoining portions of the first buriedelectrodes 515 or both. For example, a short isotropic silicon etch mayremove polycrystalline material, which may be used for the first buriedelectrodes 515, at a higher etch rate than the single crystallinesemiconductor material of the semiconductor mesas 150.

According to another embodiment, a first etch process forms the openings305 x in the capping layer 220 and stops at the first surface 101. Asecond etch step forms wide recesses 305 y by using an etch process withlower selectivity than the first etch process such that a certain amountof the first buried electrodes 515 is recessed contemporaneously withthe material of the first insulator layer 516. As a result, the width ofthe semiconductor mesas 150 can be essentially maintained such that achannel portion along the second cell trench structure 520 remainsunaffected from processes applied at the recesses 305 y.

According to another embodiment, the etch selectivity of the process forgenerating the recesses 305 y is gradually reduced with time such thatthe sidewall angles of the recess 305 y become less steep. In bothcases, the etch rate may be higher in the polycrystalline siliconmaterial, which may be used for the first buried electrodes 515, than inthe single crystalline semiconductor material of the semiconductor mesas150. Processes widening the recesses 305 y ease the later filling of therecesses 305 y with the contact material(s) without significantlyreducing the dimensions of the semiconductor mesas 150.

According to an embodiment, impurities may be implanted through thesidewalls of the recesses 305 y to reduce both a contact resistance tothe body zones 115 and the risk of latch-up effects. For example, a BF₂implant may be performed. The implant may be activated through an RTA(rapid thermal anneal) to form heavily doped contact zones 117 along thesidewall portions of the semiconductor mesas 150 exposed by the widenedrecesses 305 y. The contact zones 117 have the second conductivity typeand do not reach the second cell trench structures 520 such that avariation of a threshold voltage due to impurities of the BF₂ implantreaching the channel along the second insulator layer 526 can beavoided.

According to another embodiment, impurities may be plasma-implantedthrough the sidewalls of the widened recesses 305 v to form conformalcontact zones 117. Since the plasma implant counter-dopes portions ofthe source zones 110, the source zones 110 are provided with asufficient high net impurity concentration.

FIG. 3B shows an angled implant 380 for introducing impurities of thesecond conductivity type into exposed sidewall portions of thesemiconductor mesas 150 and the heavily doped contact zones 117 of thesecond conductivity type emerging from the angled implant 380 afteranneal. In the case of tapered recesses 305 y, the implant 380 may be anorthogonal implant perpendicular to the first surface 101. Otherwise,the implant angle with respect to the normal may be greater than 0degrees and may be directed to the second cell trench structure 520.

A barrier layer 311 may be deposited on the capping layer 220, whereinthe barrier layer 311 lines the combined contact openings 305. A mainlayer 312 is deposited that may fill the contact openings 305 completelyor that may leave voids in the contact openings 305.

FIG. 3C shows the first electrode structure 310 and the contactstructures 315 formed in the contact openings 305. A slope of thecontact openings 305 at a side oriented to the adjoining semiconductormesa 150 is steeper than a slope at the opposing side oriented to theadjoining first buried electrode 515.

FIGS. 4A to 4D illustrate a semiconductor device 500 obtained from oneof a plurality of identical semiconductor dies processed as a portion ofthe semiconductor substrate 500 a of FIG. 1A to 1D or 2A to 2E. Thesemiconductor device 500 may be a power switching device, for example,an IGBT (insulated gate bipolar transistor), e.g., a PT-IGBT (punchthrough IGBT) or an IGFET.

The semiconductor device 500 includes a semiconductor portion 100 with afirst surface 101 and a second surface 102 parallel to the first surface101. The semiconductor portion 100 is provided from a single-crystallinesemiconductor material, for example silicon Si, silicon carbide SiC,germanium Ge, a silicon germanium crystal SiGe, gallium nitride GaN orgallium arsenide GaAs. A minimum distance between the first and secondsurfaces 101, 102 is selected to achieve a specified voltage blockingcapability of the drift zone 120, for example 90 to 110 μm for a 1200 Vblocking IGBT. Other embodiments related to higher blocking devices orPT-IGBT device approaches may provide semiconductor portions 100 with athickness of several 100 μm distances between 101 and 102. Low voltageIGFETs may be thinner, e.g., at least some 10 μm.

The semiconductor portion 100 may have a rectangular shape with an edgelength in the range of several millimeters. The normal to the first andsecond surfaces 101, 102 defines a vertical direction and directionsorthogonal to the normal direction are lateral directions.

First and second cell trench structures 510, 520 extend from the firstsurface 101 into the semiconductor portion 100. The first and secondcell trench structures 510, 520 may have the same vertical dimensionsand the same lateral dimensions. According to other embodiments, thelateral and/or vertical dimensions of the first and second cell trenchstructures 510, 520 differ from each other. The vertical extension maybe in the range from 500 nm to 20 μm, e.g. from 2 μm to 7 μm. Thelateral width may be less than 2 μm, e.g. less than 1.2 μm.

The first cell trench structures 510 include first buried electrodes 515and first insulator layers 516 separating the first buried electrodes515 from the semiconductor material outside the first and second celltrench structures 510, 520. The first insulator layers 516 may have auniform thickness in a range from 50 nm to 150 nm, e.g. between 80 nmand 120 nm, by way of example. The first cell trench structures 510 mayor may not include further conductive structures, e.g. a furtherelectrode dielectrically insulated from the first buried electrodes 515.

The second cell trench structures 520 include second buried electrodes525 and second insulator layers 526 dielectrically insulating the secondburied electrodes 525 from the semiconductor material outside the firstand second cell trench structures 510, 520. The second cell trenchstructures 520 may include a further conductive structure, for example afurther electrode dielectrically insulated from the second buriedelectrodes 525. The number of first and second cell trench structures510, 520 may be equal. Other embodiments provide more first cell trenchstructures 510 than second cell trench structures 520. For example, atleast two first cell trench structures 510 are provided between twosecond cell trench structures 520, respectively. The semiconductor mesas150 between second cell trench structures 520 may or may not beconnected to the source potential.

The first and second cell trench structures 510, 520 may be parallelstripes arranged in a regular pattern. According to other embodiments,the lateral cross-sectional areas of the cell trench structures 510, 520may be circles, ellipses, ovals or rectangles, e.g. squares, with orwithout rounded corners, or rings. For example, two or three of thefirst and second cell trench structures 510, 520 may form an arrangementwith two or three concentric rings, wherein the rings may be circles,ellipses, ovals, or rectangles, e.g. squares with or without roundedcorners.

IGFET cells may be formed in the semiconductor portion 100 at a sideoriented to the first surface 101, wherein active areas of the IGFETcells are formed in first semiconductor mesas 150 a separating one firstcell trench structure 510 and one second cell trench structure 520,respectively. In the first semiconductor mesas 150 a, source zones 110of the first conductivity type may directly adjoin the first surface101. The source zones 110 form first pn junctions with body zones 115 ofthe second conductivity type, wherein interfaces between the source andbody zones 110, 115 run approximately parallel to the first surface 101at a first distance d1. The body zones 115 form second pn junctions witha drift layer 120 of the first conductivity type at a third distance d3to the first surface 101. The first and second cell trench structures510, 520 extend through the source zones 110 and the body zones 115 intothe drift layer 120.

A lateral impurity concentration profile in the source zone 110 maydecrease into the direction of the adjoining second cell trenchstructure 520. In the body zones 115 a maximum impurity concentration ofimpurities of the second conductivity type may have a distance to thefirst surface 101 that is greater than the distance between the firstsurface 101 and the first pn junctions.

The illustrated embodiment refers to a field stop IGBT and thesemiconductor portion 100 includes a pedestal layer 130 that directlyadjoins the second surface 102. The pedestal layer 130 may be acontiguous layer of the second conductivity type and may be effective asa collector layer. According to other embodiments related to, e.g.,reverse conducting IGBTs the pedestal layer 130 may include both firstportions of the first conductivity type and second portions of thesecond conductivity type, wherein the first and second portionsalternate in one lateral direction or in both lateral directions. A meannet impurity concentration in the pedestal layer 130 may be at least1×10¹⁶ cm⁻³, for example at least 5×10¹⁷ cm⁻³ to provide ohmicmetal-to-semiconductor contacts.

A second electrode structure 320 directly adjoins the second surface102. The second electrode structure 320 is electrically connected to thepedestal layer 130 and may consist of or contain, as main constituent(s)aluminum Al, copper Cu, or alloys of aluminum or copper, such as AlSi,AlCu or AlSiCu. According to other embodiments, the collector electrode320 may contain one, two, three or more sub-layers, wherein eachsub-layer contains, as main constituent(s), at least one of nickel Ni,titanium Ti, silver Ag, gold Au, tungsten W, platinum Pt and/orpalladium Pd. For example, a sub-layer may contain a metal silicide, ametal nitride, or a metal alloy containing Ni, Ti, Ag, Au, W, Pt, and/orPd. For IGBTs, the second electrode structure 320 provides a collectorelectrode that may provide or may be electrically connected to acollector terminal C of the semiconductor device 500.

In the drift layer 120, a field stop layer 128 may be provided betweenthe collector layer 130 and a drift zone 121. A mean net impurityconcentration in the field stop layer 128 may be between 5×10¹⁵ cm⁻³ and1×10¹⁷ cm⁻³. The mean net impurity concentration in the drift zone 121is lower than in the field stop layer 128. According to an embodiment,the mean net impurity concentration in the field stop layer 128 exceedsat least five times the mean net impurity concentration in the driftzone 121. The mean net impurity concentration in the drift zone 121 maybe between 5×10¹² cm⁻³ and 5×10¹⁴ cm⁻³, by way of example. For IGFETs,the pedestal layer 130 is a heavily doped contact layer of the firstconductivity type and the second electrode structure 320 provides adrain electrode that may provide or may be electrically connected to adrain terminal of the semiconductor device 500.

The second buried electrodes 525 provide insulated gate electrodes Ga. Asuitable potential applied to the insulated gate electrodes Gaaccumulates minority charge carriers in channel portions 115 a of thebody zones 115, wherein the channel portions 115 a adjoin the secondcell trench structures 520 between the source zones 110 and the driftlayer 120. If in a forward biased mode the potential applied to theinsulated gate electrodes Ga exceeds a predefined threshold voltage,inversion channels of the first conductivity type are formed in the bodyzones 115 along the second insulator layers 526, which are effective asgate dielectrics, and an on-state current flows between the source zones110 and the drift layer 120. The insulated gate electrodes Ga may beelectrically connected to a third electrode structure 330 that mayprovide or may be electrically connected or coupled to a gate terminal Gof the semiconductor device 500.

Second semiconductor mesas 150 b between first cell trench structures510 may or may not include source zones 110. In the latter case, thebody zones 115 may extend between the first surface 101 and the driftlayer 120.

The first cell trench structures 510 provide buried source electrodes Sthat may be electrically connected to an emitter terminal E of thesemiconductor device 500. The insulated gate electrodes Ga are insulatedfrom the buried source electrodes S. At least the second cell trenchstructures 520 may include a capping dielectric 210 between the firstsurface 101 and the second buried electrodes 525 to reduce an overlapbetween the insulated gate electrodes Ga and the source zones 110. Otherembodiments may provide contacts to some or all of the secondsemiconductor mesas 150 b.

A dielectric capping layer 220 may dielectrically insulate at least thesecond cell trench structures 520 and the second semiconductor mesas 150b from a first electrode structure 310 disposed at a side defined by thefirst surface 101. First contact structures 315 electrically connect thefirst electrode structure 310 with the first semiconductor mesas 150 aand such first cell trench structures 510 that directly adjoin the firstsemiconductor mesas 150 a. Second contact structures 316 electricallyconnect the first electrode structure 310 with other first cell trenchstructures 510 not directly adjoining the first semiconductor mesas 150a.

Each of the first contact structures 315 includes a first section 315 ain an opening of the capping layer 220 and a second section 315 bbetween a first semiconductor mesa 150 a and a first cell trenchstructure 510 directly adjoining the first semiconductor mesa 150 a. Thesecond section 315 b extends from the first surface 101 into thesemiconductor portion 100. A second distance d2 between the firstsurface 101 and the buried edge of the second section 315 b is greaterthan the first distance d1 and smaller than the third distance d3.

The second sections 315 b of the first contact structures 315 may haveapproximately vertical sidewalls. According to an embodiment, thesidewalls for the second sections 315 b taper with increasing distanceto the first surface 101.

According to an embodiment, first sidewalls of the second sections 315 bof the first contact structures 315 are tilted to the first surface 101and directly adjoin the first semiconductor mesas 150 a. Secondsidewalls of the second sections 315 b of the first contact structures315 may be tilted to the first surface 101 and directly adjoin the firstburied electrodes 510. The first sidewalls oriented to the firstsemiconductor mesas 150 and the second sidewalls oriented to the firstburied electrodes 510 may have identical slope angles. The secondsidewalls may deviate to a higher degree from a normal to the firstsurface 101 than the first sidewalls of the second sections 315 b of thefirst contact structures 315.

The second sections 315 b of the first contact structures 315 arelocated in the vertical projection of first sections of the firstinsulator layers 516. The first insulator layers 516 may have a uniformwidth, wherein the width of the first insulator layers 516 may be equalto or less than a width of the second sections 315 b of the firstcontact structures 315. The first contact structures 315 are deep enoughto provide a contact to the body zones 115.

Heavily doped contact zones 117 may be formed in the body zones 115 ofthe first semiconductor mesas 150 a along the interfaces to the firstcontact structures 315. The first electrode structure 310 as well as thethird electrode structure 330 may include at least one barrier layer311, 331, and a main layer 312, 332, respectively. The barrier layers311, 331 may have a uniform thickness in the range of 5 nm to 100 nm andmay consist of or include a layer of titanium nitride TiN, tantalumnitride TaN, titanium tungstenide TiW, titanium Ti or tantalum Ta, byway of example. The main layers 312, 332 may consist of or containtungsten or tungsten-based metals like titanium tungstenide TiW, heavilydoped polysilicon, carbon C, aluminum Al, copper Cu or alloys ofaluminum and copper, for example AlCu or AlSiCu.

The first and second contact structures 315, 316 may be solid contactstructures, may include a porous layer or may have voids as shown inFIG. 2. The source zones 110 may be provided as narrow stripes and mayalternate with portions of the body zones 115 in a lateral directionparallel to stripe shaped first and second cell trench structures 510,520. Accordingly, the first contact structures 315 may be stripesextending along the whole longitudinal extension of the semiconductormesas 150 or separated narrow contact structures arranged in lines alongthe longitudinal extension of the semiconductor mesas 150.

Uncertainties and inequalities of different lithographic layersresulting in a misalignment between contact structures and semiconductormesas conventionally limit a minimal mesa width at about 600 nm.Instead, the semiconductor device 500 of FIGS. 4A to 4D facilitatesnarrowing the width of the semiconductor mesas 150 down to less than 300nm, for example 200 nm and less.

Further embodiments concern layout modifications of the first and secondtrench structures 510, 520 to further reduce an effective channel widthfor increasing short-circuit ruggedness, e.g. by segmenting the secondcell trench structures 520 or by increasing locally a thickness of thesecond insulator layers 526.

FIG. 5 refers to a semiconductor device 500 which may be a powerswitching device, e.g., an IGFET or an IGBT, with stripe-shaped firstand second cell trench structures 510, 520 arranged parallel to eachother and at a regular center-to-center distance (pitch). One, two ormore, for example, four first cell trench structures 510 may be providedbetween two neighboring second cell trench structures 520. Source zones110 are provided in first semiconductor mesas 150 a on both sides ofeach second cell trench structure 520. The source zones 110 may bepatterned along a lateral direction, wherein source zones 110 assignedto the same first semiconductor mesa 150 a are separated by extensionportions of the body zone 115 in the respective first semiconductor mesa150 a. The reduced total channel width improves short-circuitruggedness.

First sections 315 a of source/body contacts in the dielectric cappinglayer 220 and second sections 315 b between first semiconductor mesas150 a and the concerned first cell trench structures 510 areself-aligned to the source zones 110.

The embodiments of FIGS. 6A to 6C refer to reinforcement implants and/orauxiliary contacts 318 in edge areas 690 of semiconductor devices 500.

The semiconductor device 500 of a FIG. 6A includes an active area 610including functional IGFET cells and an edge area 690 surrounding theactive area and devoid of functional IGFET cells. The edge area 690 mayinclude non-functional IGFET cells, for example IGFET cells without anysource zones or without source zones electrically connected to a deviceterminal, without any control electrode or without control electrodeselectrically connected to a device terminal. The edge area 690 mayinclude further structures, e.g., a HV (high voltage) terminationstructure.

The active area 610 may include stripe-shaped first and second celltrench structures 510, 520 that may be arranged parallel to each otherand at a regular center-to-center distance (pitch). One, two or more,for example, four first cell trench structures 510 may be providedbetween two neighboring second cell trench structures 520. Source zones110 are provided in first semiconductor mesas 150 a on both sides ofeach second cell trench structure 520. In each first semiconductor mesa150 a one single or a plurality of separated source zones 110 may beformed along a longitudinal extension of the first semiconductor mesas150 a. Second semiconductor mesas 150 b are formed between the firstcell trench structures 510. The first and second cell trench structures510, 520 as well as the first and second semiconductor mesas 150 a, 150b may extend into the edge area 690.

The edge area 690 includes heavily doped termination zones 170 of thesecond conductivity type. In the termination zones 170 a concentrationof impurities of the second conductivity type is high enough such thatimpurities of the first conductivity type introduced into portions ofthe termination zones 170 during formation of the source zones 110 donot completely compensate the impurities of the second conductivitytype. First sections 315 a of source/body contacts as well as firstsections 318 a of auxiliary contacts in the dielectric capping layer 220and second sections 315 b of the source/body contacts as well as secondsections 317 b of the auxiliary contacts between first semiconductormesas 150 a and the concerned first cell trench structures 510 areself-aligned to the source zones 110 in both lateral directions.

According to an embodiment, a high-dose low-energy BF₂ implant may becombined with an implant providing a conventional edge termination,wherein both implants may use the same implant mask. According toanother embodiment, a single high-dose low-energy BF₂ implant combinedwith an appropriate thermal budget provides both the conventional deepedge termination and a high impurity concentration close to the firstsurface 101, wherein the impurity concentration close to the firstsurface 101 is sufficiently high to prevent a local overcompensation bythe source implant.

Auxiliary contacts that may electrically connect the termination zones170 with a load electrode, e.g., a source or an emitter electrode may beformed contemporaneously with the source/body contacts and by using thesame mask layer as the source/body contacts. The auxiliary contactsincrease the ruggedness of the edge area 690.

In FIG. 6B the edge area 690 of a semiconductor device 500 includes anouter area 699 and a transition area 691 between the active area 610 andthe outer area 699. The active area 610 includes stripe-shaped first andsecond cell trench structures 510, 520 that may be arranged parallel toeach other and at a regular center-to-center distance (pitch). One, twoor more, for example, four first cell trench structures 510 may bearranged between two neighboring second cell trench structures 520.Source zones 110 are provided in first semiconductor mesas 150 a on bothsides of each second cell trench structure 520. Second semiconductormesas 150 b are formed between first cell trench structures 510. Thefirst and second cell trench structures 510, 520 as well as the firstand second semiconductor mesas 150 a, 150 b may extend into the edgearea 690.

The outer area 699 may include a floating termination zone 170 of thesecond conductivity type with sections formed in the first and secondsemiconductor mesas 150 a, 150 b. Source/body contacts 315 areexclusively formed within the active area 610. Auxiliary contacts 318may be formed in the transition area 691 and may extend into the outerarea 699. The auxiliary contacts 318 are provided only for secondsemiconductor mesas 150 b that do not adjoin one of the second celltrench structures 520 including gate electrodes.

Source zones 110 formed below the auxiliary contacts 318 are assigned tonon-functional IGFET cells and remain inactive. As a consequence, theauxiliary contacts 318 can be formed contemporaneously with thesource/body contacts 315 using the same lithography process and the sameetch and implant masks without providing functional IGFET cells inproximity to the outer area 699, which could adversely affect deviceperformance. The auxiliary contacts 318 improve device ruggedness in theedge area 690.

In FIG. 6C the orthogonal second cell trench structure 520 x intersectsand connects the second cell trench structures 520 in the transitionarea 691. In the lateral projection of the first and second cell trenchstructures 510, 520 third cell trench structures 530 separated by thirdsemiconductor mesas 150 c are formed on an outer side of the orthogonalsecond cell trench structure 520 x, wherein the outer side is orientedto the edge area 690. The outer area 699 may include a floatingtermination zone 170 of the second conductivity type with sectionsformed in the third semiconductor mesas 150 c.

Since none of the third cell trench structures 530 includes a gateelectrode, the edge area 690 is devoid of functional IGFET cells. Sourcezones 110 that may be formed in the transition area 691contemporaneously with the source zones 110 in the active area 610 arenon-functional. As a consequence, a contiguous auxiliary contact 318 acan be formed contemporaneously with the source/body contacts 315 usingthe same lithography process and the same etch and implant masks withoutproviding functional IGFET cells in proximity to the outer area 699. Thecontiguous auxiliary contact 318 a may extend along a complete edge ofthe active area 610.

The embodiment of FIGS. 7A and 7B distinguishes from the embodiment ofFIG. 6B in that the second buried electrodes 525 are recessed from thefirst surface 101 in an idle portion of the edge area 690. A dielectricmaterial may fill the recesses instead of the second buried electrodes525. According to the illustrated embodiment a conductive material 535insulated from the second buried electrodes 525 may fill the recesses. Adielectric structure 230 may insulate the second buried electrodes 525from the conductive material 535 which may be same as the material ofthe first buried electrodes 515. Outside the idle portion the secondburied electrodes 525 are not recessed such that contacts to the secondburied electrodes 525 may be arranged in the outer area 699 outside theidle portion.

In the idle portion of the edge area 690 the second buried electrodesare not arranged to form contiguous inversion channels. As aconsequence, the idle portion of the edge area 690 is devoid offunctional IGFET cells. Source zones 110 that may be formed in thetransition area 691 contemporaneously with the source zones 110 in theactive area 610 are non-functional. As a consequence, a contiguousauxiliary contact 318 a can be formed contemporaneously with thesource/body contacts 315 using the same lithography process and the sameetch and implant masks without providing functional IGFET cells inproximity to the outer area 699. The contiguous auxiliary contact 318 amay extend along a complete edge of the active area 610.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A semiconductor device, comprising: a first and asecond cell trench structure extending from a first surface into asemiconductor body, wherein a first semiconductor mesa separates thefirst and second cell trench structures, the first cell trench structurecomprises a first buried electrode and a first insulator layer, a firstvertical section of the first insulator layer separates the first buriedelectrode from the first semiconductor mesa, and the first semiconductormesa comprises a source zone of a first conductivity type directlyadjoining the first surface; a capping layer on the first surface; and acontact structure comprising a first section in an opening of thecapping layer and a second section in the first semiconductor mesa orbetween the first semiconductor mesa and the first buried electrode,wherein a lateral net impurity concentration of the source zone parallelto the first surface increases in the direction of the contactstructure.
 2. The semiconductor device of claim 1, wherein the sourcezone and a body zone of a second conductivity type form a pn junction ata first distance to the first surface and a vertical extension of thesecond section of the contact structure exceeds the first distance. 3.The semiconductor device of claim 2, wherein in the body zone a maximumimpurity concentration of impurities of the second conductivity type hasa greater distance to the first surface than the pn junction between thesource and body zones.
 4. The semiconductor device of claim 1, wherein aplurality of spatially separated source zones is arranged along thefirst semiconductor mesa.
 5. The semiconductor device of claim 1,further comprising: a heavily doped termination zone of the secondconductivity type in an edge area that is devoid of functional IGFETcells and that surrounds a cell area including functional IGFET cells,wherein for corresponding distances to the first surface, in thetermination zone a concentration of impurities of the secondconductivity type is higher than a concentration of impurities of thefirst conductivity type in the source zone.
 6. The semiconductor deviceof claim 1, further comprising: an auxiliary contact arranged in an edgearea that is devoid of functional IGFET cells and that surrounds a cellarea including functional IGFET cells, wherein the auxiliary contactcomprises a first section in an opening of the capping layer and asecond section extending into the semiconductor body between asemiconductor mesa and a cell trench structure.
 7. The semiconductordevice of claim 6, wherein the auxiliary contact directly adjoins secondsemiconductor mesas between first cell trench structures.
 8. Thesemiconductor device of claim 6, further comprising: an orthogonalsecond cell trench structure intersecting and connecting a plurality ofsecond cell trench structures; and third cell trench structures in thelateral projection of the first and second cell trench structures andthird semiconductor mesas in the lateral projection of the first andsecond semiconductor mesas, wherein the third cell trench structures andthe third semiconductor mesas are arranged on a side of the orthogonalsecond cell trench structure oriented to the edge area and the auxiliarycontact directly adjoins the third semiconductor mesas.
 9. Thesemiconductor device of claim 6, wherein the second buried electrodesare recessed in an idle portion of the edge area and the auxiliarycontact directly adjoins the first and second semiconductor mesas in theidle portion.